When the clays that are to be calcined have had a little of the solution of soda, potash, or saltpetre, at 5o of the pese acid, added to them, they do not easily lose their hydraulic property by too high a degree of calcination.* This mode would be embarrassing and occasion some expense; but common salt (muriate of soda) might be substituted, and would be but little expensive or troublesome. It appears probable that a like result would be obtained with sea water: because we have seen, page 68, that the Dutch make artificial trass of clay which they draw from the bottom of the sea; and which they sell as natural trass.

When the clays to be transformed are calcined with a current of atmospheric air, the mortars harden in the water much more promptly than when the clays are burned out of a current of air. I cannot assert that the resístance of such mortars is much greater, though it appears to me to be probable. As it is important to obtain, in many circumstances, mortars which harden promptly in the water: the clays on these occasions should be calcined in a current of air, in such furnaces as I have described.

Ashes, and the scoria of forges, belong to the class of artificial puzzolanas: there are some which give very good, and others which give very bad results.

Basalt, when calcined to a proper degree, affords good artificial puzzo

lanas.

Amongst mortars composed of hydraulic lime and sand, those which harden most promptly in water do not always give the greatest resistance: ' but those do, generally, which are composed of fat lime and puzzolana either natural or artificial. The most certain means of knowing whether hydraulic lime is of good quality, is to reduce it to paste with water, and to plunge it in this state, into water, to see if it hardens speedily: or, otherwise, to mix one part of the lime in paste, with two of sand. To learn the quality of hydraulic cements, or natural puzzolanas, it is proper to make a little mortar, using two parts of cement, and one of fat lime measured in paste, and place it under water.

All hydraulic mortars harden quicker in summer than in winter. If the mortar, made for trial in summer, is composed of hydraulic lime and sand, or if it be composed of hydraulic lime alone, a very good result will be certain, if at the end of eight or ten days the hardening be such that no impression can be made on the essay by pressing strongly with the finger. If this result is obtained only after the lapse of fifteen or twenty days, it is a proof that the lime is only moderately hydraulic.

With mortar made of hydraulic cement and fat lime, a similar induration should be obtained in from three to five days, if the cement be of the first quality, and if it has been calcined in a current of air. Should the hardening take place only at the end of twelve or fifteen days, with cement calcined in a current of air, it may still be employed with advantage. But if the cement has not been calcined with a free current, instead of hardening in from three to five days, it will require from twelve to fifteen, and still give a very good resistance. It may be estimated that, in winter, the induration will demand nearly twice as much time as in summer, without any diminution of the strength of the mortar.

Arenes are clays which have been subjected to the action of fire: they are therefore true natural puzzolanas. It appears that France contains this substance in many places; and it will be found in many more, as research*This is the effect, when the clays contain no lime; but I do not know whether it would be the same, if lime were present. Au.

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es shall be multiplied. It is with arenes as with natural puzzolanas and trass: all have not, by any means, the same energy: but those which are feeble may be advantageously employed in mortars to be exposed to the air, while those which have more energy may replace hydraulic cements in constructions under water, in the case of ordinary works, and where no circumstances demand a prompt induration.

The use of energetic arenes affords good mortar for masonry in water and in air, at a very cheap rate.

Concrete is nothing else than masonry made of hydraulic mortar and small stones. The use of concrete is of great advantage in founding under water at depths which occasion great expense if pumped dry. Foundations in concrete require little or no pumping: they are capable of supporting the heaviest loads.

The goodness of the concrete depends on the quality of the hydraulic mortar. It appears that the ancients sometimes constructed with this substance: its use since has been much neglected. All the advantages possible, have not yet been derived from this mode of construction; its advantages will be better appreciated when its use shall be more extended; and when the manner of perfecting hydraulic mortars shall be better understood.

Sect. 2.-ON MORTARS IN THE AIR.

CHAPTER XI.

Of Mortars made of Lime, Sand and Puzzolana.

If it is of great consequence, in making mortars that are always to remain under water, to be able to make them of good quality; it is of no less consequence to know how to prepare those which are to remain exposed to the air, in such a way that the masonry shall be lasting. People have been struck with the solidity of the remains of Roman masonry, while masonry which we ourselves have erected has often been of very limited duration. All have concurred in attributing the difference to the superiority of Roman mortars. To explain this superiority, it has been supposed that the Romans were possessed of a peculiar manner of slaking lime. Mr. Lafaye published in 1777, a method of slaking, which consisted, as described in our first section, in plunging the lime in the water for a few seconds-the lime being placed in baskets, and then to allow it to slake in the air. He gave this process as a secret recovered from the Romans; and he pretended that by this process as good mortar was obtained as theirs. This made much noise at the time; but it was soon ascertained that although there was some advantage in this mode of slaking lime, it was far from giving the mortars the superior quality the author claimed. Afterward, Mr. Loriot announced that mortar like that of the Romans might be obtained by mixing a certain quantity of powdered quick lime with lime slaked and reduced to paste in the common mode; but experience did not confirm this method of M. Loriot.

Others, to explain the goodness of Roman mortars, have attributed it to the time that has elapsed since the works were executed, whence has come the saying that "mortar is still new that is not one hundred years old," (le mortar que n'a pas cent ans est encore un enfant.) But then, it is asked, how comes it that so much masonry perishes before it is an hundred years old, while, in the same climate, some is found which has passed through near twenty centuries without repairs, and far the greater part is still standing? Those who pretend that a great lapse of time is necessary to the induration of mortars, think that the lime absorbs carbonic acid (although situated in the heart of the masonry) and thus passes to the state of carbonate; but several facts contradict this opinion: for there are certain limes which afford very good mortar in a very short time, while others never indurate, as is often seen in demolitions. On the other hand we have seen, in the first section, that according to the analysis of several ancient mortars, many were found which had very great hardness, and which nevertheless contained only a small quantity of carbonic acid; and we know that

the lime used in building is never wholly deprived of it. We cannot admit, then, that carbonic acid penetrates far into the interior of masonry; and it is proved, by multiplied observations, that moisture remains during a very long time in the interior of certain walls. Dr. John reports, on this subject, that about ten years ago, they demolished the piers of the Tower of St. Peter's, at Berlin; this tower had been built eighty years, and the pillars were twenty seven feet thick; the mortar on the outside was dry and hard, but that in the middle was as fresh as if it had been lately placed there. I can state that in 1822, that the lower part of a bastion at Strasburg, being under repair, the mortar was found to be as fresh as if just laid, and nevertheless, this bastion was erected in 1666; the revetment was only about seven feet thick, but the moisture of the earth resting against it, prevented the lower part from drying. Similar facts are observed in constructions still more ancient. It results from what has been advanced, that the good quality of the mortars of several ancient structures is not due to the manner of slaking the lime, as Mr. Lafaye supposed, nor to the process of making mortar supposed by Mr. Loriot, nor to the time that has elapsed since they were built. The experiments which follow will confirm this remark; which is in accordance also with general opinion at the present day. The great number of hydraulic works which were to De repaired or rebuilt at Strasburg, induced me to direct my first researches to the object of obtaining good mortars for the water; and it was only when these were well advanced that I began to study mortars for the air. I quitted the place before finishing my experiments. The experiments that I made, though few, appear to me, however, to throw some light on the theory of mortars in the air, and to explain in a satisfactory manner, whence was derived the good quality of the mortars found in many ancient works. It will be useful I think to report them.

It is the opinion of a great many constructors that when common, or fat lime is to be used, it is necessary to have it lie wet in vats or pits for a long time: it is asserted, that the older it is the better it is. The experiments of the following table have for object to verify this important point.

Observations on the Experiments of Table No. XXX.

To make these experiments, I took fat lime which had been slaked and lying wet in a pit for five years-a portion of the same having been used in the construction of the theatre of Strasburg. The mortars were all made

in the same manner, and broken in the same way as the hydraulic mortars; they were left in the air in a cellar for one year before cutting them down to their ultimate dimensions and submitting them to the test: the proportions of sand varied from two up to three parts of sand for one of lime measured in paste. The resulting mortars had no consistency, and crumbled between the fingers with the greatest ease. I confess I was much surprised at the result, for the sand which I used was the same as that used in the mortars of the first section; and, as I have said, was a granitic sand very slightly earthy. I made another experiment with the same lime, and the same sand washed to free it of the small quantity of earth which it contained, but I obtained no better result. I also repeated this essay with another sand, also washed, varying the quantity of water used in making the mortar, but always with results similar to those of the above table, that is to say, mortars without any consistency.

These bad results cannot be attributed to the quality of the lime, for it has been shown by the analysis in page forty-one, to be the product of a calcarious carbonate containing only a very small quantity of iron: this is the lime that has been used for a long time both for the public and private edifices of Strasburg. The theatre, as I have observed, was built of this lime: this beautiful structure does not promise, therefore, to be of long duration.

At the same time that I made the experiments of the preceding table with lime and sand, I made corresponding ones with the same lime and trass, and also with lime, sand and trass. The following table gives the results.